1. Field of the Invention
The present invention relates to a method of controlling film thickness of optical thin film when forming the film, in particular, that on the basis of optical technique, an apparatus of controlling film thickness for carrying out the method, and an apparatus of manufacturing dielectric multilayer film that is mainly used as optical thin film and can be controlled in thickness with high precision during the formation thereby. Optical thin films have been used in wider variety of optical components or elements, such as waveguide, diffraction grating, light emitter, indicator, optical memory and solar cell. In particular, as for optical thin films used for dense wavelength division multiplexing devices in communications technology field, which involves optical communications, the tendency to be made with multilayers is remarkable. Accordingly, it has become essential to control with high precision the optical film thicknesses of each layer in the multilayer structure of optical thin film.
2. Description of the Related Art
Measuring film thickness of thin film during the growth thereof is essential to control the deposition rate and the film thickness. As for optical thin films, the optical film thickness (product of the refractive index and the physical film thickness), which determines an optical property, such as reflectance and transmittance, is more useful than the physical film thickness. Therefore, widely conducted is monitoring the optical film thickness by measuring an optical property of thin film during the growth thereof according to a so-called optical film thickness controlling method, which is to measure an optical property of thin film. The optical film thickness controlling method includes a monochromatic photometry, a dichromatic photometry and a polychromatic photometry. Of these optical film thickness controlling methods, the simplest is the monochromatic photometry.
The monochromatic photometry involves making use of a peak (and also a bottom, each being equivalent to a maximum and a minimum, respectively, hereinafter) that appears when the optical film thickness of the thin film being formed reaches to an integral multiple of the λ/4 (λ: a wavelength of incident monochromatic light). Such a peak does not always appear when the optical film thickness of the film being formed reaches to an integral multiple of the λ/4 for the first time after the start of the growth, if the optical film thickness of the substrate-sided adjacent layer on which the latest surface layer film being formed is stacked on the coated surface thereof is not equal to an integral multiple of the λ/4, or if the admittance of the system including the adjacent layer cannot be shown with a mathematical real number. However, in such cases, once the peak has appeared, it periodically appears in the growth cycle of the optical film thickness coinciding to an integral multiple of the λ/4.
However, in the monochromatic photometry, the aforesaid conventional methods involving the peak control using the appearing peak cannot avoid degraded control precision to some extent in principle, because the intensity of light varies little with respect to the increasing optical film thickness in the vicinity of the peak.
The precision can be improved by using an interference filter designed for a wavelength slightly different from a desired one to be used for the control to terminate the film formation at a point, other than the vicinity of the peaks, where the intensity of light varies significantly. As an approach of this kind, the intensity of light (reciprocal transmittance), which is an optical property, may be measured to select an optical phase angle area that provides a high control precision of the growth optical film thickness, thereby determining a terminating time point for film formation (for example, see the Patent Reference 1).
On the other hand, in the art of the Patent Reference 2, for example, pursues use of a conventional monochromatic photometry using a desired wavelength. According to this approach, measured data group obtained right before the intensity of light measured (transmittance) forms a peak in response to the growth of an integral multiple of the λ/4 of optical film thickness is regressed to a quadratic function by way of the least squares method. And a time point at which a peak on the regressed function may be formed is predicted to determine a timing of terminating the film formation, accordingly. Most preferably, the timing is the predicted point itself, but if specific conditions are to be considered, the timing is determined referring to the predicted point as the time point basis.
Patent Reference 1: Japanese Patent Laid-Open No. S58-140605 (p. 2 to 3, FIG. 1)
Patent Reference 2: Japanese Patent Laid-Open No. S63-28862 (p. 2 to 6, FIGS. 1 and 2)